The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Operating an internal combustion engine under lean of stoichiometry conditions can improve fuel efficiency, but may increase NOx emissions. Known aftertreatment systems for internal combustion engines operating lean of stoichiometry can include a three-way catalytic converter upstream of a lean-NOx reduction catalyst, also referred to as a lean NOx trap (hereafter ‘LNT device’), which can be used in concert with other exhaust aftertreatment devices. Known three-way catalytic converters (‘TWC’) function to oxidize engine-out hydrocarbons (‘HC’) and carbon monoxide (‘CO’), and reduce nitrides of oxygen (‘NOx’) emissions during stoichiometric engine operation and oxidize HC and CO emissions during lean operation.
A LNT device adsorbs NOx emissions during lean engine operation and preferably operates within about a 250° C. to 450° C. temperature range with effectiveness decreasing above and below that temperature range. Effectiveness of known LNT devices can be reduced due to exposure to elements present in fuel, including sulfur. A LNT device requires periodic regeneration to desorb and reduce adsorbed NOx elements. Regenerative techniques can include operating the spark-ignition engine at an air/fuel ratio that is rich of stoichiometry for a period of time.
During engine operation, an oxygen storage capacity (hereafter ‘OSC’) in the TWC can become saturated with oxygen during lean engine operation. During an initial portion of the regeneration process, oxygen stored in the TWC oxidizes reductants (e.g. HC, CO, H2) emitted by the engine and emits the processed mixture to the LNT device. This air/fuel mixture causes the LNT device to desorb stored NOx. As the reductants are consumed in the TWC during the initial portion of the regeneration process, desorbed NOx emissions may escape.
Known regeneration methods include running the engine rich of stoichiometry to generate reductants which react with stored NOx to regenerate the LNT. A significant portion of the reductants is consumed by the oxygen stored in the TWC. Additionally, the rich air/fuel ratio necessary to neutralize the oxygen stored in the TWC results in a delivery delay of reductants to the LNT further decreasing NOx reduction efficacy.